[RISCV] Validate the SEW and LMUL operands to __builtin_rvv_vsetvli(max)

These are required to be constants, this patch makes sure they
are in the accepted range of values.

These are usually created by wrappers in the riscv_vector.h header
which should always be correct. This patch protects against a user
using the builtin directly.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D102086